Wavelength
Division Multiplexing -WDM-is a method of transmitting data from
different sources over the same fiber optic link at the same time
whereby each data channel is carried on its own unique wavelengthover
a fiber optic cable.Typically WDM is using to increase signalling
and bandwidth capacity over singlemode fiber optic cable, The
result is a link with an aggregate bandwidth that increases with
the number of wavelengths employed. In this way WDM technology
can maximize the use of the fiber optic infrastructure that is
available; what would normally require two or more fiber links
instead requires only one. Dense Wave Division Multiplexing (DWDM)
combines up to 64 wavelengths onto a single fiber. DWDM technology
uses an ITU standard 100GHz or 200GHz spacing between the wavelengths,
arranged in several bands at ~1500-1600nm. With DWDM technology,
the wavelengths are close together (compared to CWDM), meaning
that transponders are generally more complex and expensive than
CWDM. However, with DWDM, the advantage is a much higher density
of wavelengths, and also longer distances, especially with MRV's
low dispersion solution for the Fiber Driver-LD system.

WDM
technology is made up of a number of building blocks. When put
together into a network, these blocks provide a complete solution
for multi-wavelength services, and bandwidth enhancement over
a single or dual fiber strand..

Optical
Budget
With most WDM systems, the key to distance is calculating optical
budget (the amount of light on the fiber). There are several key
parameters that affect the optical budget:

Transmit
Power: how much power is transmited by the laser Receive Sensitivity: how much power the receiver needs
in order to have valid signal Transit Loss: loss along the fiber-optic cable (per km)
Insertion Loss: loss across a multiplexer when inserting
the channel into a trunk Pass thru Loss: loss accross an add-drop multiplexer for
a trunk signal passing through Removal Loss: loss accross a demultiplexer when removing
the channel from the trunk

Transmit
Power:

How
much power is transmited by the laser
Receive Sensitivity: how much power the receiver needs in order
to have valid signal
Transit Loss: loss along the fiber-optic cable (per km)
Insertion Loss: loss across a multiplexer when inserting the channel
into a trunk
Passthru Loss: loss accross an add-drop multiplexer for a trunk
signal passing through
Removal Loss: loss accross a demultiplexer when removing the channel
from the trunk

Dispersion:
Even when the optical budget is within the parameters of the optical
power, there is another issue to consider, which is dispersion
(the spreading out of light along a fiber). CWDM systems are typically
limited by optical budget to around 100km. DWDM systems, due to
higher power and better sensitivity, can operate to 200km typically,
and are limited by dispersion. MRV provides low dispersion DWDM
modules with the Fiber Driver-LD product over 600km.

WDM
technology is made up of a number of building blocks. When put
together into a network, these blocks provide a complete solution
for multi-wavelength services, and bandwidth enhancement over
a single or dual fiber strand.

Multiplexing
is the process of combining several data streams operating at
unique wavelengths on several fibers onto a single fiber "trunk".
Demultiplexing is just the opposite, taking a fiber trunk and
separating it into individual fibers, each transporting a different
wavelength signal.

An
Add/Drop Multiplexer takes a single wavelength from a trunk, pulls
the signal out, and allows a new signal at the same wavelength
to be inserted into the trunk at (roughly) the same spot. All
the other wavelengths pass through the add/drop mux with only
a small loss of power (usually a few dB).

This
concept is especially important when planning WDM networks, and
plays an important role in the overall distance a WDM network
can span.

Sub-rate
Multiplexing is the process of placing several data streams onto
a single wavelength, in an effort to further increase the number
of data streams in a WDM system. There are several mechanisms
of sub-rate multiplexing:
TDM - Time Domain Multiplexing (SONET and many proprietary versions)
FDM - Frequency Domain Multiplexing (e.g., QAM)
Statistical Multiplexing (Ethernet or other Layer-2)

This
example shows how multiple channels of ESCON can be converted
with the Fiber Driver-LD ESCON Solution into a single wavelength
before wave division multiplexing, allowing up to 64 ESCON channels
on a 16 wavelength CWDM system, or hundreds of ESCON channels
using DWDM. The LD product also offers 2-Port Gigabit Ethernet
Multiplexing modules.

A
cross-connect is a device that allows an administrator to select
which port will have a connection to which other port or ports.
This example shows the configurability of the NC316-XP Media Cross
Connect. Cross-connects have several uses in WDM systems:
Fault-tolerant links - a cross-connect can be programmed to switch
from a primary to a secondary link on the event of a loss of optical
power (or other signals), allowing a simple means of creating
protection-switching. With SFP-based cross-connects, this function
operates in the same device as the transponder.

Reconfigurable networks - a cross-connect can be configured and
re-configured depending on the needs of the network. This programming
can be set by time-of-day as well to maximize the appropriate
utilization of the wavelengths. For example, 3 wavelengths may
be used during the day for data, 1 for storage, but at night and
on weekends, during massive backup operations, two of the wavelengths
can be switched to storage usage.

Trunk switching - a cross-connect can be programmed to configure
several 'colored' SFP ports at the same time, so that, if these
ports are all connected to the same Mux/Demux, the cross-connect
acts as a trunk-switch, allowing the whole trunk (that is, the
whole set of wavelengths) to move to a different fiber.